Efficient Multicast Association to Improve the Throughput in IEEE 802.11 WLAN

This paper deals with the problem of optimal association of stations (S T A s) to access points (A P s) for mulicast services in IEEE 802.11 WLAN. In a multicast session, all the subscribed S T A s receive the multicast data packet at the same data rate (R _{m i n} ) from their respective serving A P s. A higher value of R _{m i n} improves the multicast throughput by completing the ongoing multicast session in lesser time. This also improves the unicast throughput as the cycle duration is shared by the unicast and multicast sessions. To provide multicast services to the S T A s, we need to select a minimum cardinality subset of A P s as the system message overhead depends on this cardinality. However, such a minimum cardinality subset of A P s may not be possible to activate simultaneously due to the limited number of available orthogonal frequency channels. In this paper, we develop a combined greedy algorithm that selects a subset of A P s with minimum cardinality for which a conflict-free frequency assignment exists and finds an association between the S T A s and the selected A P s that maximizes the R _{m i n} value. Through simulation we have shown that the proposed algorithm selects significantly less number of A P s for different R _{m i n} values in comparison to the well-known metrics for multicast association like RSSI, minimum hop-distance, normalized-cost and in-range STA number.     

Study of Deep Levels in a HIT Solar Cell

The results of studying a HIT (heterojunction with an intrinsic thin layer) Ag/ITO/a-Si:H(p)/a-Si:H(i)/c-Si(n)/a-Si:H(i)/a-Si:H(n⁺)/ITO/Ag solar cell by the capacitance–voltage characteristic and current deep-level relaxation transient spectroscopy methods are presented. The temperature dependence of the capacitance–voltage characteristics of the HIT structure and deep-energy-level parameters are studied. The results of comprehensive studies by the above methods are used to determine the features of the energy-band diagram of actual HIT structures.     

Impact of deployment size on the asymptotic capacity for wireless ad hoc networks under Gaussian channel model

We study the throughput capacity and transport capacity for both random and arbitrary wireless networks under Gaussian Channel model when all wireless nodes have the same constant transmission power P and the transmission rate is determined by Signal to Interference plus Noise Ratio (SINR). We consider networks with n wireless nodes \(\{v_1,v_2,\ldots,v_n\}\) (randomly or arbitrarily) distributed in a square region B _a with a side-length a. We randomly choose n _s node as the source nodes of n _s multicast sessions. For each source node v _i , we randomly select k points and the closest k nodes to these points as destination nodes of this multicast session. We derive achievable lower bounds and some upper bounds on both throughput capacity and transport capacity for both unicast sessions and multicast sessions. We found that the asymptotic capacity depends on the size a of the deployment region, and it often has three regimes.     

HierHybNET: Cut-set upper bound of ad hoc networks with cost-effective infrastructure

This paper introduces an information-theoretic upper bound on the capacity scaling law for a hierarchical hybrid network (HierHybNET), consisting of both n wireless ad hoc nodes and m base stations (BSs) equipped with l multiple antennas per BS, where the communication takes place from wireless nodes to a remote central processor through BSs in a hierarchical way. We deal with a general scenario where m, l, and the backhaul link rate scale at arbitrary rates relative to n. Then, a generalized cut-set upper bound under the HierHybNET model is derived by cutting not only the wireless connections but also the wired connections. In addition, the corresponding infrastructure-limited regime is identified.     

Electronic Processes in CdIn<Subscript>2</Subscript>Te<Subscript>4</Subscript> Crystals

The results of investigations of electrical, optical, and photoelectric properties of CdIn₂Te⁴ crystals, which were grown by the Bridgman method are presented. It is shown that electrical conductivity is determined mainly by electrons with the effective mass m_n = 0.44m₀ and the mobility 120–140 cm²/(V s), which weakly depends on temperature. CdIn₂Te₄ behaves as a partially compensated semiconductor with the donor-center ionization energy E_d = 0.38 eV and the compensation level K = N_a/N_d = 0.36. The absorption-coefficient spectra at the energy hν < E_g = 1.27 eV are subject to the Urbach rule with a typical energy of 18–25 meV. The photoconductivity depends on the sample thickness. The diffusion length, the charge-carrier lifetime, and the surface-recombination rate are determined from the photoconductivity spectra.     

Spectral photosensitivity of <Emphasis Type="Italic">a</Emphasis>-SiGe:H/<Emphasis Type="Italic">c</Emphasis>-Si heterostructures

Characteristics of a-SiGe:H/c-Si heterostructures produced by rapid plasma-chemical low-frequency (55 kHz) deposition are studied. High photosensitivity of a-SiGe:H films is established. The spectral position of the maximum of photosensitivity of a a-SiGe:H/c-Si heterostructure can be varied from 830 to 920 nm by increasing the content of germanium in an a-SiGe:H alloy and decreasing its band gap.     

A study of the effect of oxygen on the intensity of erbium photoluminescence in amorphous SiO<Subscript>x</Subscript>:(H,Er) films formed by DC magnetron sputtering

The effect of oxygen on the intensity of erbium photoluminescence at λ=1.54 μm in amorphous a-SiO_x:H(Er) films formed by dc magnetron sputtering was studied. The oxygen content in the gaseous phase ranged from 0.1 to 12 mol %, with other parameters of deposition remaining constant. Analysis of an a-Si:(H, Er, O) system showed that the range of homogeneity of amorphous a-SiO_x:H(Er) is retrograde (T=const). The range of homogeneity can be conventionally divided into two portions, each of which should contain either of two differently charged [Er-O] ⁿ⁻ and [Er-O-Si-O] ^m− clusters (m>n). This inference is confirmed experimentally: in the range of oxygen concentrations amounting to 5.5–8 mol % in the plasma, unusual associative processes take place probably directly above the growing film surface; these processes are caused by the appearance of [Er-O-Si-O] ^m− clusters in the plasma and at the surface. It is these processes that account for the intensification of erbium photoluminescence as the oxygen content increases above 5.5 mol %. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 37, No. 7, 2003, pp. 853–859. Original Russian Text Copyright ? 2003 by Undalov, Terukov, Gusev, Kudoyarova.     

Spectra of low-temperature photoluminescence in thin polycrystalline CdTe films

The band of intrinsic (e–h) radiation emission by the subsurface potential barriers of crystal grains and the edge doublet band arising as LO-phonon replicas of the e–h band are observed in the spectra of the low-temperature (4.2 K) photoluminescence of fine-grained (with a grain size of d_cr ≤ 1 µm) CdTe films. Film doping with the In impurity results in quenching of the doublet band, while heat treatment leads to activation of the intrinsic band, a short-wavelength shift of the red boundary (ΔE_r = 16–29 meV) and the halfwidth modulation (Δ_A = 6–17 meV) of which correlate with the height of micropotential barriers and the temperature of recombining hot photocarriers.     

Electrical properties of <Emphasis Type="Italic">n-</Emphasis>GaN/<Emphasis Type="Italic">p</Emphasis>-SiC heterojunctions

Epitaxial GaN layers were grown by hydride vapor phase epitaxy (HVPE) on commercial (CREE Inc., USA) p⁺-6H-SiC substrates (Na ? Nd ≈ 7.8 × 10¹⁷ cm^s?3) and n⁺-6H-SiC Lely substrates with a predeposited p⁺-6H-SiC layer. A study of the electrical properties of the n-GaN/p-SiC heterostructures obtained confirmed their fairly good quality and demonstrated that the given combination of growth techniques is promising for fabrication of bipolar and FET transistors based on the n-GaN/p-SiC heterojunctions.     

Effect of annealing temperature and rate of sputtering on the magnetic properties and microstructure of the polycrystalline nickel films with (200) texture

The dependences of the magnetic properties and morphology of polycrystalline nickel (Ni) films with the (200) texture that are fabricated using the dc magnetron sputtering on the SiO₂/Si(100) substrates on sputtering rate annealing temperature T, and film thickness d are analyzed. It is demonstrated that an increase in the sputtering rate from 17 to 35 nm/min does not affect the saturation magnetization 4πM and ferromagnetic resonance line width ΔH but leads to a significant increase in the coercivity H _c for the films whose thickness d is greater than critical thickness d _cr (d > d _cr). It is also demonstrated that d _cr depends on both sputtering rate and annealing temperature. The films with the thickness d > d _cr exhibit the stripe domain structure whose period increases with increasing d and rate v. The annealing of the films with d ≥ 40 nm at T ≈ 200–400°C results in an increase in ΔH and H _c by a factor of 2–4, an increase in 4πM by 25%, an increase in grain size ξ by a factor of 20–30, and the formation of the stripe domain structure in the films that do not exhibit such structure prior to annealing and substantial strengthening of the (200) texture.     

Electron mobility in the inversion layers of fully depleted SOI films

The dependences of the electron mobility μ_eff in the inversion layers of fully depleted double–gate silicon-on-insulator (SOI) metal–oxide–semiconductor (MOS) transistors on the density N _e of induced charge carriers and temperature T are investigated at different states of the SOI film (inversion–accumulation) from the side of one of the gates. It is shown that at a high density of induced charge carriers of N _e > 6 × 10¹² cm^–2 the μeff(T) dependences allow the components of mobility μ_eff that are related to scattering at surface phonons and from the film/insulator surface roughness to be distinguished. The μ_eff(N _e ) dependences can be approximated by the power functions μ_eff(N _e) ∝ N _e ^?n . The exponents n in the dependences and the dominant mechanisms of scattering of electrons induced near the interface between the SOI film and buried oxide are determined for different N _e ranges and film states from the surface side.     

Study of photocapacitance in diodes fabricated from silicon doped with vanadium

The levels of vanadium in the band gap of n-and p-Si were determined using photocapacitance measurements. It is shown that vanadium introduces levels only in the upper half of the band gap of n-Si; these levels have ionization energies of about E _c ?0.21 eV, E _c ?0.32 eV, and E _c ?0.52 eV. By contrast, V levels are located both in the upper and lower halves of the p-Si band gap: E _c ?0.26 eV, E _v +0.52 eV, E _v +0.42 eV, and E _v +0.31 eV. It is ascertained that the photoionization cross sections of all vanadium levels are larger for electrons than for holes. It is shown that the concentration of electrically active vanadium centers in n-and p-Si depends on both the concentration of shallow-level impurities and the time of vanadium diffusion into Si.     

Special features of radiation-defect annealing in silicon <Emphasis Type="Italic">p-n</Emphasis> structures: The role of Fe impurity atoms

Deep-level transient spectroscopy is used to study the formation of complexes that consist of a radiation defect and a residual impurity atom in silicon. It is established that heat treatment of the diffused Si p⁺-n junctions irradiated with fast electrons lead to the activation of a residual Fe impurity and the formation of the FeVO (E_0.36 trap) and FeV₂ (H_0.18 trap) complexes. The formation of these traps is accompanied by the early (100–175°C) stage of annealing of the main vacancy-related radiation defects: the A centers (VO) and divacancies (V₂). The observed complexes are electrically active and introduce new electron (E_0.36: E _t ^e =E _c -0.365 eV, σ _n =6.8×10^?15 cm²) and hole (H_0.18: E _t ^h =E _v +0.184 eV, σ _p =3.0×10^?15 cm²) levels into the silicon band gap and have a high thermal stability. It is believed that the complex FeVO corresponds to the previously observed and unidentified defects that have an ionization energy of E _t ^e =E _c ?(0.34–0.37) eV and appear as a result of heat treatment of irradiated diffused Si p⁺-n junctions.     

Dissociation energies of a C<Subscript>i</Subscript>C<Subscript>s</Subscript> complex and the <Emphasis Type="Italic">A</Emphasis> center in silicon

Reactions of annealing of A centers (VO) and complex centers consisting of interstitial carbon atoms and substitutional carbon atoms (C_iC_s) in n-Si irradiated with fast electrons or ⁶⁰Co gamma quanta were analyzed. The kinetics of isochronous annealing of the above centers was evaluated. By comparing the results of calculations with published experimental data, which were obtained by measuring the concentration of centers with the level E_c?0.17 eV in the course of isochronous heat treatment of irradiated n-Si, the dissociation energy E for the C_iC_s complex was estimated at 1.10±0.05 eV. The activation energy for annealing of this center was found to be equal to \(E_{aC_i C_s } \approx 2.0eV\). The dissociation energy for the A center was estimated at E_A=1.94 eV.     

Effect of the magnetic phase transition on the charge transport in layered semiconductor ferromagnets TlCrS<Subscript>2</Subscript> and TlCrSe<Subscript>2</Subscript>

TlCrS₂ and TlCrSe₂ crystals were synthesized by solid-state reaction. X-ray diffraction analysis showed that TlCrS₂ and TlCrSe₂ compounds crystallize in the hexagonal crystal system with lattice parameters a = 3.538 Å, c = 21.962 Å, c/a ≈ 6.207, z = 3; a = 3.6999 Å, c = 22.6901 Å, c/a ≈ 6.133, z = 3; and X-ray densities ρ _x = 6.705 and 6.209 g/cm³, respectively. Magnetic and electric studies in a temperature range of 77–400 K showed that TlCrS₂ and TlCrSe₂ are semiconductor ferromagnets. Rather large deviations of the experimental effective magnetic moment of TlCrS₂ (3.26 μ_B) and TlCrSe₂ (3.05 μ_B) from the theoretical one (3.85 μ_B) are attributed to two-dimensional magnetic ordering in the paramagnetic region of strongly layered ferromagnets TlCrS₂ and TlCrSe₂. The effect of the magnetic phase’s transition on the charge transport in TlCrS₂ and TlCrSe₂ is detected.     

High-voltage (900 V) 4 <Emphasis Type="Italic">H</Emphasis>-SiC Schottky diodes with a boron-implanted guard <Emphasis Type="Italic">p-n</Emphasis> junction

High-voltage (900 V) 4H-SiC Schottky diodes terminated with a guard p-n junction were fabricated and studied. The guard p-n junction was formed by room-temperature boron implantation with subsequent high-temperature annealing. Due to transient enhanced boron diffusion during annealing, the depth of the guard p-n junction was equal to about 1.7 μm, which is larger by approximately 1 μm than the projected range of 11 B ions in 4H-SiC. The maximum reverse voltage of fabricated 4H-SiC Schottky diodes is found to be limited by avalanche breakdown of the planar p-n junction; the value of the breakdown voltage (910 V) is close to theoretical estimate in the case of the impurity concentration N = 2.5 × 10¹⁵ cm^?3 in the n-type layer, thickness of the n-type layer d = 12.5 μm, and depth of the p-n junction r _j = 1.7 μm. The on-state diode resistance (3.7 mΩ cm²) is controlled by the resistance of the epitaxial n-type layer. The recovery charge of about 1.3 nC is equal to the charge of majority charge carriers that are swept out of an epitaxial n-type layer under the effect of a reverse voltage.     

Characterization of an <Emphasis Type="Italic">a</Emphasis>-Si:H/<Emphasis Type="Italic">c</Emphasis>-Si interface by admittance spectroscopy

The capabilities of admittance spectroscopy for the investigation of a-Si:H/c-Si heterojunctions are presented. The simulation and experimental results, which compare very well, show that the admittance technique is sensitive to the parameters of both the a-Si:H layer and the a-Si:H/c-Si interface quality. In particular, the curves showing capacitance versus temperature have two steps, accompanied by two bumps in the temperature dependence of the conductance. The first step, occurring in the low temperature range (100–200 K), is related to the transport and response of gap states in the a-Si:H layer. The second step, occurring at higher temperatures (>200 K), is caused by a carrier exchange with interface states and appears when the interface defect density exceeds 5 × 10¹² cm^?2. Then, the interface defects affect band bending, and, thus, the activation energy of de-trapping, which favors exchange with electrons from a-Si:H and holes from c-Si, respectively, for an increasing defect density.     

Method for narrowing the directional pattern of an InGaAs/GaAs/AlGaAs multiwell heterolaser

A semiconductor laser with a new waveguide is developed. It allows significant narrowing of the directional pattern (to 4° in the plane perpendicular to the p–n junction). In the used waveguide, the minimum excess of the effective refractive index n_eff of the excitation mode over the substrate refractive index n_s (n_eff–n_s ? 1) is provided by selecting the thickness of Al_0.3Ga_0.7As confinement layers, which significantly increases the waveguide mode size and leads to directional-pattern narrowing.     

A 4<Emphasis Type="Italic">H</Emphasis>-SiC<Emphasis Type="Italic">p-i-n</Emphasis> diode fabricated by a combination of sublimation epitaxy and CVD

The possibility of fabricating heavily doped (N _a ?N _d ≥ 1 × 10¹⁹ cm_?3) p⁺-4H-SiC layers on CVD-grown lightly doped n-4H-SiC layers by sublimation epitaxy has been demonstrated. It is shown that a Au/Pd/Ti/Pd contact, which combines a low specific contact resistance (～2 × 10^?5 Ω cm²) with high thermal stability (up to 700°C), is the optimal contact to p-4H-SiC. The p-n structures obtained are used to fabricate packaged diodes with a breakdown voltage of up to 1400 V.     

Effect of the Ti-Nanolayer Thickness on the Self-Lift-off of Thick GaN Epitaxial Layers

The effect of the type of substrate, sapphire substrate (c- and r-orientation) or GaN/Al₂O₃ template (c- and r-orientations), on the nitridation of an amorphous titanium nanolayer is shown. The effect of the titanium-nanolayer thickness on thick GaN epitaxial layer self-separation from the substrate is revealed. The titanium-nanolayer thickness at which thick GaN layer is reproducibly self-separated is within 20–40 nm.